Method and apparatus to fold optics in tools for measuring shape and/or thickness of a large and thin substrate

ABSTRACT

A semiconductor measuring tool has a folding mirror configuration that directs a light beam to pass the same space multiple times to reduce the size and footprint. Furthermore, the folding mirrors may reflect the light beam at less than forty-five degrees; thereby allowing for smaller folding mirrors as compared to the prior art.

PRIORITY

The present application claims the benefit under 35 U.S.C. §120(pre-AIA) of U.S. patent application Ser. No. 13/561,377, filed Jul.30, 2012, issued as U.S. Pat. No. 9,279,663, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention is directed generally toward semiconductor waferprocessing and more particularly toward measuring the shape andthickness of semiconductor wafers.

BACKGROUND OF THE INVENTION

In a typical tool for measuring the shape and thickness of a siliconwafer, two channels of interferometers are employed to measure bothsurfaces of the wafer. Each interferometer usually comprises lenses thatimage the wafer to a video camera. This way the whole wafer can bemeasured by the camera with millions of pixels, eliminating the need tomechanically scan the wafer, and the throughput is dramatically improvedcompared to scanning systems.

One disadvantage of this method is that the size of the measuring toolis large due to the size of imaging optics. As the semiconductorindustry shifts to larger wafers (for example from 300 mm to 450 mm) thesize of the measuring tool may increase significantly. Simply scaling upexisting measuring tools designed for a 300 mm wafer to accommodate a450 mm wafer would result in a measuring tool much more expensive andfifty percent larger in every direction. At that size, the measuringtool may not physically fit in a space currently designated for suchmeasuring tools.

Consequently, it would be advantageous if an apparatus existed that issuitable for measuring the shape and thickness of a silicon wafer with acompact optical arrangement.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a novel method andapparatus for measuring the shape and thickness of a silicon wafer witha compact optical arrangement.

In one embodiment of the present invention, a measuring device includesmirrors for directing the optical path along an axis parallel to an axisnormal to the surface of the wafer. Such configuration allowsutilization of the space along the length of the measuring tool.

In another embodiment of the present invention, a method for measuringsemiconductor wafers includes reflecting an interferometric image from afirst axis normal to a surface of the semiconductor wafer to a secondaxis parallel to the first axis where the interferometric image iscaptured by a camera. By this method, the optical path from the wafer tothe image is extended concurrently with the length of the measuringtool. Increased length allows for larger optical components andtherefore imaging larger semiconductor wafers.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention claimed. The accompanyingdrawings, which are incorporated in and constitute a part of thespecification, illustrate an embodiment of the invention and togetherwith the general description, serve to explain the principles.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous objects and advantages of the present invention may bebetter understood by those skilled in the art by reference to theaccompanying figures in which:

FIG. 1 shows a perspective view of a measuring tool;

FIG. 2 shows a perspective view of a measuring tool according to thepresent invention; and

FIG. 3 shows a flowchart of a method for measuring a semiconductorwafer.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings. The scope of theinvention is limited only by the claims; numerous alternatives,modifications and equivalents are encompassed. For the purpose ofclarity, technical material that is known in the technical fieldsrelated to the embodiments has not been described in detail to avoidunnecessarily obscuring the description.

Referring to FIG. 1, a perspective view of a measuring tool is shown.The measuring tool may include one or two reference flats 102, 120.Reference flats 102, 120 are the references for measuring surfaceflatness of a semiconductor wafer 100. The measuring tool may alsoinclude one or two collimators 104, 122. Collimators 104, 122 narrow orcollimate light from a semiconductor wafer 100. The measuring tool mayalso include one or more series of folding mirrors 106, 108, 110, 124,126, 128.

Each series of folding mirrors 106, 108, 110, 124, 126, 128 may reflectan interferometric image of a semiconductor wafer 100 from a collimator104, 122 to one or more optical elements including a camera 118, 136.Folding mirrors 106, 108, 110, 124, 126, 128 allow an interferometricimage of a semiconductor wafer 100 to be redirected so that themeasuring tool may be made more compact and stable.

A first or large folding mirror 106, 124 may reflect an interferometricimage from a collimator 104, 122 along a path substantiallyperpendicular to the path of the interferometric image when it exits thecollimator 104, 122. A second or mid-folding mirror 108, 126 may reflectthe interferometric image from the large folding mirror 106, 124 along apath substantially perpendicular to both the path of the interferometricimage when it exits the collimator 104, 122 and the path of theinterferometric image when it is reflected by the large folding mirror106, 124. A third or small folding mirror 110, 128 may reflect theinterferometric image from the mid-folding mirror 108, 126 along a pathsubstantially parallel to the path of the interferometric image when itis reflected by the large folding mirror 106, 124. In thisconfiguration, an interferometric image of a semiconductor wafer 100 maybe extended over a necessary distance defined by the parameters of theoptics used to transmit the interferometric image and the size of thesemiconductor wafer 100.

The small folding mirror 110, 128 may reflect the interferometric imagefrom the mid-folding mirror 108, 126 to a series of optics. The seriesof optics may include a λ/4 plate 112, 130, a polarized beam splitter114, 132, a relay lens 116, 134 and a camera 118, 136. The λ/4 plate112, 130 is an optical device that alters the polarization state of thebeam from circularly polarized to linearly polarized so that the beampasses through the polarized beam splitter 114, 132 with minimum loss.The relay lens 116, 134 may be either a lens or group of lenses thatre-constructs the interferometric image at the camera 118, 136.

The polarized beam splitter 114, 132 is also the point at which anillumination source (not shown) is introduced. Light from anillumination source may enter the polarized beam splitter 114, 132 fromone side, become linearly polarized after reflected by the polarizedbeam splitter, and then pass through the λ/4 plate 112, 130 where thelight may become circularly polarized. The light may then reflect offeach of the folding mirrors 106, 108, 110, 124, 126, 128 to enter thecollimator 104, 122 and reference flat 102, 120. The light may thenilluminate the semiconductor wafer 100. An interference pattern may beformed between the reference flat 102, 120 and the semiconductor wafer100. The interference pattern is the image delivered to the camera 118,136.

Semiconductor inspection facilities may be configured to accommodate ameasuring tool designed for 300 mm semiconductor wafers. A 450 mmsemiconductor wafer may require a measuring tool with correspondinglarger optics. For example, a measuring tool suitable for a 450 mmsemiconductor wafer may include one or two reference flats 102, 120fifty percent larger than reference flats 102, 120 suitable for ameasuring tool designed to accommodate a 300 mm semiconductor wafer 100(a 450 mm semiconductor wafer 100 being fifty percent larger than a 300mm semiconductor wafer 100). The size of the semiconductor wafer 100being inspected may dictate the distance that the light from anillumination source needs to travel in order to expand and illuminatethe entire semiconductor wafer 100. Likewise, the same distance may benecessary to focus the interferometric image.

In a measuring tool according to FIG. 1, all of the dimensions of themeasuring tool may need to be extended to accommodate a 450 mmsemiconductor wafer. Such dimensions may increase the size of themeasuring tool beyond the space available in existing 300 mmsemiconductor processing facilities. Furthermore, optics such as mirrorsmay be prone to vibration and gravitational distortion. Increasing thesize of certain optical components may make such optical components moreprone to these effects.

Referring to FIG. 2, a perspective view of a measuring tool according tothe present invention is shown. The measuring tool may include one ortwo reference flats 202, 220 and one or two collimators 204, 222. Themeasuring tool may also include one or more series of folding mirrors206, 208, 210, 224, 226, 228.

Each series of folding mirrors 206, 208, 210, 224, 226, 228 may reflectan interferometric image of a semiconductor wafer 200 from a collimator204, 222 to one or more optical elements including a camera 218, 236.Folding mirrors 206, 208, 210, 224, 226, 228 allow an interferometricimage of a semiconductor wafer 200 to be redirected so that themeasuring tool may be made more compact and stable.

A first or large folding mirror 206, 224 may reflect an interferometricimage from a collimator 204, 222 along a path toward a second ormid-folding mirror 208, 226. The large folding mirror 206, 224 may beoriented with respect to an axis normal to the reference flats 202, 220such that the angle of incidence of light exiting the collimator 204,222 may be less than 45°; an angle of incidence less than 45° may allowthe large folding mirror 206, 224 to comprise a smaller, lighter mirroras compared to the prior art. The mid-folding mirror 208, 226 mayreflect the interferometric image from the large folding mirror 206, 224toward a third or small folding mirror 210, 228. The mid-folding mirror208, 226 may be positioned relative to the large folding mirror 206, 224so as to redirect the light to travel between the large mirror and thecollimator 204, 222.

The small folding mirror 210, 228 may reflect the interferometric imagefrom the mid-folding mirror 208, 226 along a path substantially parallelto the path of the interferometric image when it exited the collimator204, 222. In this configuration, a dimension of the measuring toolcorresponding to an axis normal to a semiconductor wafer 200 beingmeasured may define a distance that may be utilized multiple times tofocus the interferometric image. Elongated the measuring tool along thatdimension may increase the distance traveled by the interferometricimage by some multiple of the actual elongation, and thereby reduce thescaling factor when processing 450 mm semiconductor wafers 200 ascompared to 300 mm semiconductor wafers 200. Furthermore, the compactnature of the measuring tool may reduce the potential for vibrationaldistortion.

The small folding mirror 210, 228 may reflect the interferometric imagefrom the mid-folding mirror 208, 226 to a series of optics. The seriesof optics may include a λ/4 plate 212, 230, a polarized beam splitter214, 232, a relay lens 216, 234 and a camera 218, 236. The λ/4 plate212, 230 is an optical device that alters the polarization state of thebeam from circularly polarized to linearly polarized so that the beampasses the polarized beam splitter 214, 232 with minimum loss. The relaylens 216, 234 may be either a lens or group of lenses that re-constructsthe interferometric image at camera 218, 236.

The polarized beam splitter 214, 232 is also the point at which anillumination source (not shown) is introduced. Light from anillumination source may enter the polarized beam splitter 214, 232 fromits side, be reflected by it towards λ/4 plate 212, 230. At this point,it becomes linearly polarized. After passing through the λ/4 plate 212,230, the light may become circularly polarized. The light may thenreflect off of each of the folding mirrors 206, 208, 210, 224, 226, 228to enter the collimator 204, 222 and reference flat 202, 220. Thecircularly polarized light may then illuminate the semiconductor wafer200. An interference pattern may be formed between the reference flat202, 220 and the semiconductor wafer 200. The interference pattern isthe image to be delivered to the camera 218, 236.

Semiconductor inspection facilities may be configured to accommodate ameasuring tool designed for 300 mm semiconductor wafers. One potentialadvantage of the present invention is the ability to operate a measuringtool for 450 mm semiconductor wafers in a facility designed for 300 mmsemiconductor wafers. A 450 mm semiconductor wafer may require ameasuring tool with correspondingly larger optics. A measuring toolsuitable for a 450 mm semiconductor wafer may include one or tworeference flats 202, 220 fifty percent larger than reference flats 202,220 suitable for a measuring tool designed to accommodate a 300 mmsemiconductor wafer 200 (a 450 mm semiconductor wafer 200 being fiftypercent larger than a 300 mm semiconductor wafer 200). The size of thesemiconductor wafer 200 being inspected may dictate the distance lightfrom an illumination source needs to travel in order to expand andilluminate the entire semiconductor wafer 200. Likewise, the samedistance may be necessary to focus the interferometric image. Ameasuring tool with folding mirrors according to the present invention206, 208, 210, 224, 226, 228 may conform to size restrictions imposed bya facility designed for 300 mm semiconductor wafers 200 even when themeasuring tool is configured to inspect 450 mm semiconductor wafers 200.One skilled in the art may appreciate that the concepts set forth hereinare equally applicable to semiconductor wafer measuring tools of allsizes, and that the example of 450 mm semiconductor wafers is exemplaryand should not be considered a limitation.

Referring to FIG. 3, a flowchart of a method for measuring asemiconductor wafer is shown. The method may include illuminating 300 asemiconductor wafer with an illuminator. The light may pass through areference flat prior to illuminating the semiconductor wafer to producean interference pattern. The interferometric image may provide importantmeasurement information about the semiconductor wafer. The image mayneed to travel a certain distance to be reduced in size to be capturedby a camera. Such distance, if applied in any one direction, may beimpractical for a measuring tool in a semiconductor production facility.The distance may be distributed along various dimensions within themeasuring tool.

The interferometric image may travel along a path substantially normalto the surface of the semiconductor wafer, and may then be reflected 302by a large folding mirror. The interferometric image may be reflected302 by the large folding mirror so as to direct the interferometricimage along one or more first alternative dimensions of the measuringtool; furthermore, the interferometric image may be reflected 302 by thelarge folding mirror at an angle less than 45°.

The interferometric image may be reflected 304 by a mid-folding mirroralong a path defined by one or more second alternative dimensions of themeasuring tool. The interferometric image may then be reflected 306 by asmall folding mirror along a path substantially parallel to a pathdefined by a line normal to the surface of the semiconductor wafer. Theinterferometric image may then be captured 308 by a camera suitable forcapturing an interferometric image of a semiconductor wafer.

By this method, any increase in the length of the measuring tool alongan axis corresponding to a line normal to the surface of a semiconductorwafer may be reflected multiple times in distance travelled by theinterferometric image. Furthermore, a measuring tool such as the oneshown in FIG. 2 may measure both sides of a semiconductor wafersimultaneously. In that case, optics dedicated to each side of thesemiconductor wafer may be configured to utilize different areas of themeasuring tool so as to achieve the necessary distance traveled by eachinterferometric image. Two separate interferometric image paths mayutilize one dimension of the measuring tool multiple times such that anyincrease in the length of the measuring tool along an axis correspondingto that dimension may be reflected multiple times in distance travelledby each interferometric image. The interferometric image may therebypass the same space, for example the space between the collimator andthe large mirror, multiple times, reducing the size of the measuringtool overall and placing components of the measuring tool in proximityto increase stability.

These systems and methods may allow for measuring tools capable ofmeasuring larger semiconductor wafers that prior art measuring toolscannot. They may also allow for the use of smaller optics and improvedstability.

It is believed that the present invention and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, construction,and arrangement of the components thereof without departing from thescope and spirit of the invention or without sacrificing all of itsmaterial advantages. The form herein before described being merely anexplanatory embodiment thereof, it is the intention of the followingclaims to encompass and include such changes.

What is claimed is:
 1. A system for directing an interferometric imagein a semiconductor wafer measuring tool, comprising: a first foldingmirror oriented to reflect an interferometric image from a first surfaceof a semiconductor wafer, an axis from the semiconductor wafer to thefirst folding mirror defining a first dimension; a second folding mirrororiented to reflect the interferometric image from the first foldingmirror, an axis from the first folding mirror to the second foldingmirror defining a second dimension; a third folding mirror oriented toreceive the interferometric image from the second folding mirror, anaxis from the second folding mirror to the third folding mirror defininga third dimension; and an optical assembly oriented to receive theinterferometric image from the third folding mirror, an axis from thethird mirror to the optical assembly defining a fourth dimension,wherein: the first dimension and the fourth dimension are substantiallyparallel; and the optical assembly disposed on a side of thesemiconductor wafer opposite the first surface such that an increase ina distance between the semiconductor wafer and the first folding mirroralso produces a corresponding increase in a distance between the thirdfolding mirror and the optical assembly.
 2. The system of claim 1,wherein the first folding mirror is configured to reflect aninterferometric image of a semiconductor wafer larger than 400 mm. 3.The system of claim 1, wherein the first folding mirror is configured toreflect an interferometric image at an angle of reflection less than45°.
 4. The system of claim 1, further comprising: a fourth foldingmirror oriented to reflect an opposing interferometric image from thesemiconductor wafer, an axis from the semiconductor wafer to the fourthfolding mirror corresponding to the first dimension; a fifth foldingmirror oriented to reflect the opposing interferometric image from thefourth folding mirror, an axis from the fourth folding mirror to thefifth folding mirror corresponding to the second dimension; and a sixthfolding mirror oriented to receive the opposing interferometric imagefrom the fifth folding mirror, an axis from the fifth folding mirror tothe sixth folding mirror corresponding to the third dimension.
 5. Thesystem of claim 4, wherein the first folding mirror is oriented toreflect the interferometric image of a first surface of a semiconductorwafer; and the fourth folding mirror is oriented to reflect the opposinginterferometric image of a second surface of the semiconductor wafer. 6.An interferometric measuring tool for measuring the shape and thicknessof a semiconductor wafer, comprising: a first folding mirror oriented toreflect an interferometric image from a first surface of a semiconductorwafer, an axis from the semiconductor wafer to the first folding mirrordefining a first dimension; a second folding mirror oriented to reflectthe interferometric image from the first folding mirror, an axis fromthe first folding mirror to the second folding mirror defining a seconddimension; a third folding mirror oriented to receive theinterferometric image from the second folding mirror, an axis from thesecond folding mirror to the third folding mirror defining a thirddimension; and an optical assembly oriented to receive theinterferometric image from the third folding mirror, an axis from thethird mirror to the optical assembly defining a fourth dimension,wherein: the first dimension and the fourth dimension are substantiallyparallel; and the optical assembly disposed on a side of thesemiconductor wafer opposite the first surface such that an increase ina distance between the semiconductor wafer and the first folding mirroralso produces a corresponding increase in a distance between the thirdfolding mirror and the optical assembly.
 7. The measuring tool of claim6, wherein the first folding mirror is configured to reflect aninterferometric image of a semiconductor wafer larger than 400 mm. 8.The measuring tool of claim 6, wherein the first folding mirror isconfigured to reflect an interferometric image at an angle of reflectionless than 45°.
 9. The measuring tool of claim 6, further comprising: atleast one reference flat configured to produce an interference image inconjunction with a semiconductor wafer when the semiconductor wafer isilluminated by an illuminator; and at least one collimator lensconfigured to reduce the size of the interferometric image produced bythe reference flat and a semiconductor wafer, wherein the reference flatand collimator are interposed between an area configured to hold asemiconductor wafer for inspection and the first folding mirror.
 10. Themeasuring tool of claim 6, further comprising at least one opticalassembly comprising: a beam splitter configured to: allow an illuminatorto introduce light to the wave plate; and receive an interferometricimage; an optical relay configured to receive the interferometric imagefrom the beam splitter; and a camera configured to capture theinterferometric image from the optical relay.
 11. The measuring tool ofclaim 6, further comprising: a fourth folding mirror oriented to reflectan opposing interferometric image from the semiconductor wafer, an axisfrom the semiconductor wafer to the fourth folding mirror correspondingto the first dimension; a fifth folding mirror oriented to reflect theopposing interferometric image from the fourth folding mirror, an axisfrom the fourth folding mirror to the fifth folding mirror correspondingto the second dimension; and a sixth folding mirror oriented to receivethe opposing interferometric image from the fifth folding mirror, anaxis from the fifth folding mirror to the sixth folding mirrorcorresponding to the third dimension.
 12. The measuring tool of claim11, wherein the first folding mirror is oriented to reflect theinterferometric image of a first surface of a semiconductor wafer; andthe fourth folding mirror is oriented to reflect the opposinginterferometric image of a second surface of the semiconductor wafer.13. The measuring tool of claim 6, wherein the first dimension and thesecond dimension are substantially perpendicular.
 14. A method fordirecting an interferometric image in a semiconductor measuring tool,comprising: reflecting an interferometric image from a first surface ofa semiconductor wafer with a first folding mirror to a second foldingmirror, an axis from the semiconductor wafer to the first folding mirrordefining a first dimension; reflecting the interferometric image fromthe first folding mirror with the second folding mirror to a thirdfolding mirror, an axis from the first folding mirror to the secondfolding mirror defining a second dimension; and reflecting theinterferometric image from the second folding mirror with a thirdfolding mirror to an optical assembly, an axis from the second foldingmirror to the third folding mirror defining a third dimension, and anaxis from the third folding mirror to the optical assembly defining afourth dimension, wherein: the first dimension and the fourth dimensionare substantially parallel; and the optical assembly disposed on a sideof the semiconductor wafer opposite the first surface such that anincrease in a distance between the semiconductor wafer and the firstfolding mirror also produces a corresponding increase in a distancebetween the third folding mirror and the optical assembly.
 15. Themethod of claim 14, further comprises capturing the interferometricimage with a camera wherein the optical assembly comprises the camera.16. The method of claim 14, wherein the first folding mirror isconfigured to reflect an interferometric image at an angle of reflectionless than 45°.
 17. The method of claim 14, further comprisingilluminating the semiconductor wafer with an illumination source. 18.The method of claim 14, wherein the first folding mirror is configuredto reflect an interferometric image of a semiconductor wafer larger than400 mm.
 19. The method of claim 14, further comprising: reflecting aninterferometric image from a semiconductor wafer with a fourth foldingmirror to a fifth folding mirror, an axis from the semiconductor waferto the fourth folding mirror corresponding to the first dimension;reflecting the interferometric image from the fourth folding mirror withthe fifth folding mirror to a sixth folding mirror, an axis from thefourth folding mirror to the fifth folding mirror corresponding to thesecond dimension; and reflecting the interferometric image from thefifth folding mirror with the sixth folding mirror to an opticalassembly, an axis from the fifth folding mirror to the sixth foldingmirror corresponding to the third dimension and an axis from the sixthfolding mirror to the optical assembly defining the fourth dimension.20. The method of claim 14, wherein the first dimension and the seconddimension are substantially perpendicular.